It is known that structural discontinuities, such as holes and notches, can cause localized stress concentration in a component or structure. When the applied stress is sufficiently high the stress concentration results in cracks forming in the component or structure, which propagate from the structural discontinuity. Excessive cracking can ultimately lead to failure of the component or structure.
In many structures, such as aircrafts and bridges, holes and fastener assemblies are used to connect components within the structure. A hole site is a structural discontinuity at which failure can first be observed. Early detection of component failure can prevent catastrophic failure. Planning for early detection can be used during the design phase to minimize redundancy within a structure, and thus the overall weight of the structure.
Visual detection of surface cracking that propagates from a hole in a component or structure through which a fastener assembly is disposed can be performed by removing the fastener assembly from the hole. This can be a labor intensive task. Further, the actual removal of a fastener assembly may damage the component or structure and provide the source of a crack. Alternatively, cracks can be observed when they extend beyond the outer edge of the fastener assembly. However, it is to be appreciated that by such a time the crack may already be several millimeters long.